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United States Patent |
6,127,744
|
Streeter
,   et al.
|
October 3, 2000
|
Method and apparatus for an improved micro-electrical mechanical switch
Abstract
A method, device and circuit which applies an electrostatic repulsion
pushing force to a MEM switch armature during an opening process. The
repulsive force adds to the spring restoration force on the armature,
increasing the opening speed of the switch and aids in overcoming stiction
effects. The inventive switch includes a contact electrically connected to
a first terminal of the switch. A throw is electrically connected to a
second terminal of the switch. Finally, a mechanism is provided for
opening the switch by electrostatically causing the throw to disengage the
contact. In the illustrative implementation, the mechanism for opening the
switch includes a first charge storage structure mounted on the throw and
a second charge storage structure mounted in proximity to the first charge
storage structure. When charges are applied between the first and the
second charge storage structures, a force of repulsion is created or a
force of attraction is created depending on the polarity of the potential.
Inventors:
|
Streeter; Robert D. (Fort Wayne, IN);
McMillan; Lee A. (Fort Wayne, IN)
|
Assignee:
|
Raytheon Company (Lexington, MA)
|
Appl. No.:
|
198669 |
Filed:
|
November 23, 1998 |
Current U.S. Class: |
307/125; 200/181 |
Intern'l Class: |
H01H 057/00 |
Field of Search: |
307/125
200/181
361/234
|
References Cited
U.S. Patent Documents
5051643 | Sep., 1991 | Dworsky et al. | 200/181.
|
5278368 | Jan., 1994 | Kasano et al. | 200/181.
|
5367136 | Nov., 1994 | Buck | 200/181.
|
5578976 | Nov., 1996 | Yao | 200/181.
|
5619177 | Apr., 1997 | Johnson et al. | 337/140.
|
5677823 | Oct., 1997 | Smith | 361/234.
|
5771321 | Jun., 1998 | Stern | 385/31.
|
6016092 | Jan., 2000 | Qui et al. | 333/262.
|
6040611 | Mar., 2000 | De Los Santos et al. | 257/415.
|
6046659 | Apr., 2000 | Loo et al. | 200/181.
|
6057520 | May., 2000 | Goodwin-Johansson | 200/181.
|
Primary Examiner: Jackson; Stephen W.
Assistant Examiner: Roberto; Rios
Attorney, Agent or Firm: Daly, Crowley & Mofford, LLP
Claims
What is claimed is:
1. An MEM switch comprising:
a cantilever beam having first and second ends;
a cantilever support member coupled to the second end of the cantilever
beam;
a first insulative layer disposed on the cantilever beam;
a first control surface disposed on the first insulative layer such that
the first control surface, the first insulative layer and the cantilever
beam form a first capacitor having a substantially fixed capacitance;
a first conductive surface opposing the cantilever beam;
a second insulative layer disposed on the first conductive surface; and
a second control surface disposed on the second insulative surface and
facing the first control surface such that the second control surface, the
second dielectric layer, and the first conductive surface form a second
capacitor having a substantially fixed capacitance, and the first and
second control surfaces form a third capacitor having a variable
capacitance that is determined by the position of the cantilever beam.
2. The switch according to claim 1, wherein the cantilever beam, the
cantilever support member, and the first conductive surface are connected
so as to be at the same electrical potential.
3. An MEM switch, comprising:
a cantilever support member;
a cantilever beam having first and second ends, the first end being coupled
to the cantilever support member;
a contact member spaced from the cantilever support member for contacting
the second end of the cantilever beam;
a first conductive surface opposing the cantilever beam;
a first dielectric layer disposed on the cantilever beam;
a first control surface disposed on the first dielectric layer, the first
control surface for receiving a first control voltage;
a second dielectric layer disposed on first conductive surface; and
a second control surface disposed on the second dielectric layer such that
that the first and second control surfaces face each other, the second
control surface for receiving a second control voltage, wherein respective
polarities of the first and second control voltages control the position
of the cantilever beam for actively opening and actively closing the
switch.
4. The switch according to claim 3, wherein the cantilever beam, the
cantilever support member, and the first conductive surface are connected
so as to be at the same electrical potential.
5. A method of controlling a micromechanical switch, comprising:
applying a first control voltage having a first polarity to a first control
surface spaced from a cantilever beam by a first insulative layer, such
that the first control voltage charges a first capacitor formed by the
first control surface, the first insulative layer and the cantilever beam;
applying a second control voltage having a second polarity, which is
opposite to that of the first control voltage, to a second control surface
spaced from a first conductive layer by a second insulative layer such
that the second control voltage charges a second capacitor formed by the
second control surface, the second insulative layer and the first
conductive layer for causing an end of the cantilever beam to touch a
contact so as to close the switch; and
switching a polarity of one of the first and second control voltages so as
to open the switch.
6. The method according to claim 5, further including placing cantilever
beam, a cantilever support member and the first conductive surface at
substantially the same electrical potential.
7. The method according to claim 5, further including providing
substantially equal and opposite polarity voltages on the first and second
control voltages when closing the switch.
8. The method according to claim 5, further including providing
substantially equal and same polarity voltages on the first and second
control voltages when opening the switch.
9. The method according to claim 5, further including connecting a low
impedance circuit to at least one of the first and second capacitors for
reducing a capacitor charging time.
10. The method according to claim 5, further including providing DC
voltages on the first and second control voltages.
11. The method according to claim 5, further including providing AC
voltages on the first and second control voltages.
12. The method according to claim 5, further including setting the first
control voltage to about zero volts when closing the switch.
13. The method according to claim 5, further including setting the second
control voltage to about zero volts when closing the switch.
14. The method according to claim 5, further including maintaining same
polarity signals on the first and second control voltages after the switch
is open.
15. The method according to claim 5, wherein the first control voltage is
about positive fifty volts with respect to the cantilever beam, the
cantilever support member, and the first conductive surface when closing
the switch.
16. The method according to claim 15, wherein the second control voltage is
about negative fifty volts with respect to the cantilever beam, the
cantilever support member, and the first conductive surface when closing
the switch.
17. The method according to claim 5, wherein movement of the cantilever
beam towards the contact interrupts an optical path.
18. The method according to claim 5, further including coupling a load to
the contact.
19. The method according to claim 18, further including coupling a load
voltage to the load.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical and electronic circuits and
components. More specifically, the present invention relates to
micro-electromechanical (MEM) switches.
2. Description of the Related Art
A MEM switch is a switch operated by an electrostatic charge, thermal,
piezoelectric or other actuation mechanism and manufactured using
micro-electromechanical techniques. A MEM switch may control electrical,
mechanical, or optical signal flow. Current MEM switches are usually
single pole, single throw (SPST) configurations having a rest state that
is normally open. Application of an electrostatic charge to the control
electrode (or opposite polarity electrostatic charges to a two-electrode
design) will create an attractive electrostatic force ("pull") on the
switch causing the switch to close. Currently, the switch opens by removal
of the electrostatic charge on the control electrode(s), allowing the
mechanical spring restoration force of the armature to open the switch.
Unfortunately, electrostatically actuated MEM switches constructed in
accordance with conventional teachings typically close faster than they
open and often suffer "stiction" effects that hinder the opening process
or cause the switch to fail closed. Hence, the opening and closing
("transition speed") of conventional MEM switches has been limited.
For many current applications, there is a need to increase the transition
speed of a MEM switch. Hence, there is a need in the art for further
improvements in MEM switches.
SUMMARY OF THE INVENTION
The need in the art is addressed by the MEM design and method of the
present invention. In accordance with the inventive teachings, an
electrostatic force of repulsion is used to effect operation of a switch
from a first state to a second state.
In an illustrative electrical implementation, the MEM switch includes a
contact electrically connected to a first terminal of the switch. A throw
is electrically connected to a second terminal of the switch. Finally, a
mechanism is provided for opening the switch by electrostatically causing
the throw to disengage the contact.
In the illustrative implementation, the mechanism for opening the switch
includes a first charge storage structure mounted on the throw and a
second charge storage structure mounted in proximity to the first charge
storage structure. When charges are placed on the first charge storage
structure and simultaneously on the second charge storage structure, a
force of repulsion (for equal polarity and magnitude) or attraction (for
opposite polarity and equal magnitude) is created on the throw. The
potential between the two charge storage structures depends on the charge
polarities. The charges will also have a potential value referenced to the
rest of the MEM switch structure.
Hence, the invention provides a method, device and circuit which applies an
electrostatic repulsion pushing force to an MEM switch armature during an
opening process. The repulsion force adds to the spring restoration force
on the armature, increasing the opening speed of the switch and aids in
overcoming stiction effects.
Although the inventive teachings are disclosed with respect to an
electrical application, the present teachings may be used for optical,
acoustic and other applications as will be appreciated by those skilled in
the art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a conventional electrostatically operated
micro-electromechanical (MEM) switch.
FIG. 2 is a schematic diagram of a typical setup using a conventional MEM
switch.
FIG. 3 is a sketch of the control voltage applied to a conventional MEM
switch illustrating optimum driving waveform for MEM switch closure.
FIG. 4 is a waveform for enhanced conventional MEM switch opening.
FIG. 5 is a diagram of an improved MEM switch constructed in accordance
with the teachings of the present invention.
FIG. 6 is a schematic diagram of an electrical circuit using the improved
MEM switch of the present invention.
FIG. 7 illustrates exemplary waveforms for driving the improved MEM switch
of the present invention.
DESCRIPTION OF THE INVENTION
Illustrative embodiments and exemplary applications will now be described
with reference to the accompanying drawings to disclose the advantageous
teachings of the present invention.
While the present invention is described herein with reference to
illustrative embodiments for particular applications, it should be
understood that the invention is not limited thereto. Those having
ordinary skill in the art and access to the teachings provided herein will
recognize additional modifications, applications, and embodiments within
the scope thereof and additional fields in which the present invention
would be of significant utility.
FIG. 1 is a diagram of a conventional micro-electromechanical (MEM) switch.
The switch 10' has an insulating support substrate 12' which serves as a
base. A conductive contact 14' is mounted on one side of the substrate and
a cantilever support 16' is mounted on the other. The cantilever support
16' supports a conductive cantilever beam 20'. A conductive control
surface 18' is mounted on the substrate 12' between the conductive contact
14' and the cantilever support 16'.
The switch 10' is shown as a normally open (NO) switch contact structure.
The open gap between the conductive contact 14' and the cantilever beam
20' is usually a few microns (1/1,000,000 meter) wide. When the switch
closes, the cantilever beam 20' is in direct electrical contact with the
conductive contact 14'.
A slightly larger gap exists between the cantilever beam 20' and the
conductive control surface 18'. The larger gap keeps the conductive
control surface from contacting the cantilever beam in normal operation.
The total device is usually less than 1 mm in length, width and height. If
the far end (the unattached end) of the cantilever beam is deflected
toward the conductive contact, the cantilever beam will make ohmic contact
with the conductive contact and be physically stopped by the contact. The
cantilever beam will not make electrical or physical contact with the
control surface unless the beam is physically forced to make contact with
the control surface.
In normal operation, the MEM switch 10' is drawn closed by placing an
electric charge on the conductive control surface 18'. An opposite
polarity charge is induced on the inner surface of the conductive
cantilever beam 20' opposite to the control surface. This electric charge
pair induces an electrostatic (coulomb) attractive force between the two
surfaces. If the coulomb force is strong enough to overcome the spring
restorative force of the cantilever beam, the coulomb force will deflect
the beam toward the control surface and the conductive contact. This will
close the switch. The potential between control surface 18' and the rest
of the MEM switch determines the charge pair in accordance with Q=CV,
where Q is the charge, C is the capacitance between the control surface
and the cantilever beam, and V is the applied control voltage V.sub.c.
FIG. 2 is a schematic diagram of a typical setup using a conventional MEM
switch. Those skilled in the art will appreciate that numerous circuit
configurations are permitted using MEM switches. In FIG. 2, a control
voltage supply 22' provides a control voltage V.sub.c to the control
surface 18'. The control voltage V.sub.c induces a charge on the control
surface in accordance with the relation Q=CV. The load voltage V.sub.1,
supplied by a load voltage supply 24', allows current to flow through the
load (delivering power to the load) when the MEM switch closes.
The cantilever beam restorative force is linearly proportional to the
displacement from the rest position of the beam. A small coulomb force
will make a small deflection of the beam, but will not cause the beam to
fully deflect to the conductive contact. As the coulomb force raises by
increasing the electric charge on the control surface, the beam deflection
will increase. Additionally, the coulomb force is inversely proportional
to the square of the separation distance between the charges. Thus, as the
coulomb force draws the beam closer to the contact, the distance between
the charged surfaces will decrease. This decreased distance between the
charges will increase the coulomb force on a square-law basis without a
proportional increase in the electric charge stored on the control
surface. This action identifies a minimum switching charge magnitude
(voltage) on the control surface that results in a continuous net
attraction of the cantilever beam toward the control surface.
Additionally, since the coulomb force (above the minimum required) is
increasing on a square law basis while the cantilever spring restorative
force opposing the coulomb force is only increasing linearly, the
cantilever beam experiences a continuously increasing acceleration as it
approaches the control surface and the conductive contact.
As long as the coulomb force exceeds the spring restorative force at every
point in the deflection of the beam, the MEM switch will continue to
close, and the beam will accelerate as it approaches the conductive
contact.
It should be obvious that the maximum coulomb force exists as the
cantilever beam touches the conductive contact and is at its closest
approach to the control surface. The switch will be held closed by this
maximum force. It should also be apparent that there is a maximum coulomb
charge that can be applied to the control surface. An excess charge may
result in an arc if the dielectric electric withstanding field strength
(between the control surface and any other MEM switch structure) is
exceeded or in an excess deflection of the cantilever beam causing it to
touch the control surface. Either of these actions will result in a
discharge of the electrostatic charge and the elimination of the coulomb
force holding the MEM switch closed.
Those skilled in the art may consider placing an insulating layer of
material on control surfaces 30' and 34' to prevent contact arc or
discharge of the control surfaces if they touch (as shown in FIGS. 5 and
6). The question of surface charges on such an insulator layer must be
addressed in their impact on the coulomb force between the two charged
control surfaces 30' and 34'.
Another possible configuration to eliminate the problems that occur when
control surfaces 30' and 34' physically touch is to put a wide mesh grid
of insulation material on one control surface, leaving most of the control
surfaces exposed but preventing mechanical contact between surfaces 30'
and 34'.
The application of a fixed direct current (D.C.) voltage V.sub.c to the
control surface is the simple case of the control waveform driving the MEM
switch to the closed position. The voltage V.sub.c may be of either
polarity, although the physical structure and the materials used in
fabrication of the MEM switch may favor one polarity over the other. The
application of the voltage to the control surface in the circuit of FIG. 2
will cause a charging current to flow briefly in the control circuit
(consisting of the control surface 18', the beam 20' and the control
voltage supply 22'). This charging current can be thought of as charging
the capacitor formed by the control surface and the cantilever beam. The
control voltage V.sub.c must be above the minimum (for switch closure) and
below the maximum (to prevent arcing) for the physical structure of the
MEM switch if proper operation is to occur.
Ideally, when the control surface is charged, the voltage V.sub.c may be
removed. This is usually not done due to leakage effects in the MEM switch
and the changes that occur during switch closure.
It is also possible to reverse the control polarity while the switch is
held closed so long as the time needed to discharge and recharge the
control surface with the opposite polarity charge is much less than the
time required for the cantilever beam to move sufficiently to disconnect
the load circuit. The use of polarity reversal drive can create a
mechanical wiping action on the load circuit contacts while the switch is
closed and may be a benefit for certain contact metallurgy. Polarity
reversal may also be useful in association with certain MEM fabrication
methods.
The speed of switch closure can be controlled to some degree by controlling
the magnitude of the charge on the control surface. The initial charge
placed on the control surface will have a maximum value set primarily by
arc limitations between the control surface and the remaining elements of
the MEM switch. This maximum charge will apply a maximum coulomb force to
the cantilever beam, resulting in a maximum initial closure rate for the
beam. As the beam approaches the control surface during closure, the
capacitance will increase. While application of a constant voltage will
result in an increase in the surface charge (through Q=CV) and a resultant
increase in the coulomb force beyond the square-law effect, the voltage
may exceed the capabilities of the beam to control surface spacing. Thus,
it may be necessary to reduce the control voltage as the switch closes to
prevent a discharge arc of the stored charge. This leads to a decayed
voltage waveshape versus time for the fastest MEM switch closure
operation. The shape of the control voltage decay for the fastest closure
depends on the constructional details of the MEM switch. When the MEM
switch achieves closure, the control voltage must remain between the
maximum closed voltage (determined by arc or cantilever deflection) and
the minimum voltage needed to provide sufficient coulomb force needed to
overcome the cantilever spring restoration force.
FIG. 3 is a sketch of the control voltage applied to a conventional MEM
switch illustrating optimum driving waveform for MEM switch closure. The
opening of a conventional MEM switch depends exclusively on the spring
restoration force in the cantilever beam. Since the coulomb force holding
the switch closed is stronger than the spring restoration force, the
coulomb force must be reduced to a value as close to zero as possible to
allow a maximum of opening force from the cantilever beam spring effect.
The opening force may also need to overcome "stiction" effects resulting
from the Casmir effect of from minor contact adhesion effects. While the
control surface charge can be allowed to leak off slowly, this will delay
the opening of the MEM switch. The opening speed can be improved by
accelerating the discharge of the control surface charge. The proper
selection of control voltage waveform can speed the opening process.
The conventional method to open the MEM switch 10' is to force the control
surface voltage (relative to the cantilever beam) to zero volts through a
low impedance source. A short circuit between the control surface and the
cantilever beam is the optimum means of swiftly reducing the control
voltage to zero. The electrical effect is to rapidly discharge the
capacitor formed by the control surface and the inner surface of the
cantilever beam.
A method to further enhance the discharge of the control surface charge is
to supply an opposite charge burst prior to driving the control surface
voltage to zero. The waveform to accomplish this is shown in FIG. 4.
FIG. 4 is a waveform for enhanced conventional MEM switch opening. However,
this method does have some inherent limitations in that inserting less
than the correct opposite charge will not provide the best reduction of
coulomb force. Even worse, inserting an excess opposite charge will simply
recharge the control surface and cantilever beam with opposite polarity
and delay the full reduction of the coulomb force. The speed at which this
discharge mechanism can operate is determined by the complex impedance of
the control surface current path, the cantilever beam current path (which
is the other side of the capacitor storing the surface charges) and the
external circuit for the control signal.
The method of the present invention speeds the time required to open a MEM
switch constructed in accordance with the teachings provided herein. The
inventive switch provides a coulomb repulsion force which assists the
cantilever spring force in opening the switch. The two forces act in the
same direction and are additive. The teachings of the present invention
may allow for switch opening to occur over a shorter time span than is
required to close the MEM switch. The opposite condition often occurs in
conventional MEM switches. Whereas the conventional MEM switch has only
the "pull" closing electrostatic force, the improved switch structure of
the present invention has a "push" operation by which an electrostatic
coulomb force aids in opening the switch.
FIG. 5 is a diagram of an improved MEM switch constructed in accordance
with the teachings of the present invention. As shown in FIG. 5, the
improved switch 10 includes an insulating substrate 12, a conductive
contact 14, a cantilever support 16, a first conductive surface 18 and a
cantilever beam 20 as per the conventional switch 10' of FIG. 1. However,
in accordance with the present teachings, a first control surface 30 is
provided on the lower surface 31 of the beam 20. The first control surface
30 is insulated from the beam 20 by a layer of insulation 32. A second
control surface 34 is disposed over the first conductive surface 18 and is
separated therefrom by a layer of insulative material 36.
The physical improvements illustrated in FIG. 5 over the conventional MEM
switch structure illustrated in FIG. 1 are the charge storage structure on
the cantilever beam 20 (consisting of the conductive beam 20, the first
control surface 30 and the dielectric insulator 32 therebetween) and the
addition of a charge storage structure (consisting of the first conductive
surface 18, the second control surface 34 and the insulator 36
therebetween) as a replacement to the conductive control surface of the
conventional MEM switch of FIG. 1. The two charge storage additions are
used to induce the equal and like polarity charges that will result in a
repulsive coulomb force during the opening process as discussed more fully
below. The inventive structure allows for the creation of coulomb forces
of either attraction or repulsion as will be appreciated by those skilled
in the art. The continuous mechanical connection between the cantilever
beam, the cantilever support, the first conductive surface 18 and the
support substrate allows the coulomb forces to be properly placed on the
cantilever beam.
Those skilled in the art will also recognize that the teachings of the
present invention may be realized in additional structural designs that
allow one to create two controllable charge storage areas capable of
exerting both attractive and repulsive coulomb forces on the cantilever
beam. In addition, in certain applications, i.e., optical, the first
control surface 30 and insulation 32' may not be required. The first
control surface 30 serves to facilitate the electrostatic repulsive force
on the beam 20 while isolating the electrical circuit to which the beam 20
is connected on closure of the switch 10. Beam 20' would then have to be
insulated from conductor 18' for proper operation.
A variable capacitor is formed by the two control surfaces 30' and 34' and
the dielectric (air, vacuum, inert gas, etc.) therebetween. This capacitor
must be considered in addition to the two fixed capacitors (34, 36, 18)
and (30, 32, 20) formed by the control surfaces mentioned above. The
variable capacitance is minimum when the switch is open and is maximum
when the switch is closed.
The closing process for the improved MEM switch 10 is the same as that of
the conventional MEM switch 10'. Opposite polarity charges are induced on
control surfaces 30' and 34'. The opposite charges create an attractive
coulomb force as discussed above. These forces must overcome the
cantilever spring restorative force, and there is a minimum charge that
allows switch closure. There is also a maximum charge and associated
voltage that can be tolerated as discussed above.
FIG. 6 is a schematic diagram of an electrical circuit using the improved
MEM switch of the present invention. Those skilled in the art will
appreciate that numerous circuit configurations are permitted using the
improved MEM switch of the present invention. In FIG. 6, a first control
voltage supply 21 provides a control voltage V.sub.c1 to the first control
surface 30 on the cantilever beam 20 and a second control voltage supply
22 provides a second control voltage V.sub.c2 to the second control
surface 34. Note the depiction of the equivalent capacitors C1, C2 and C3
representing the fixed capacitor provided by the first control surface 30
and the beam 20, the second control surface 34 and first conductive area
18, and the variable capacitance between the first and second control
surfaces 30 and 34, respectively. Charging the control surfaces may be
considered to be equivalent to charging the capacitors C1, C2 and C3. The
closure speed depends on the time it takes to charge these capacitors. In
all probability, equivalent capacitors C1 and C2 will dominate the
charging process. Low impedance circuit are required to minimize the
charging time of capacitors C1 and C2.
Opposite charges can be induced on the two control surfaces by applying
opposite voltage polarities to the two control surfaces as connected in
FIG. 6. While the maximum attractive force will result from having the two
charges on capacitors C1 and C2 equal and opposite, there need not be
charge equality on the two control surfaces. Further, there need not be a
specific voltage polarity on the first or the second control surface.
Nonetheless, a specific implementation may favor a defined polarity
structure or a continuous variation of polarity in the driving signals.
As discussed above with respect to the conventional MEM switch of FIG. 2,
the application of fixed direct current (D.C.) voltages V.sub.c1 and
V.sub.c2 to the control surfaces 30 and 34 of the improved MEM switch 10
of the present invention is the simple case of the control waveform
driving the MEM switch 10 to the closed position. Voltages V.sub.c1 and
V.sub.c2 may be combined (for closure) into an isolated voltage V.sub.c
connected between 30 and 34 or (in the conventional switch) 18 and 20. The
voltage V.sub.c may be of either polarity, although the physical structure
and the materials used in fabrication of the MEM switch may favor one
polarity over the other. The application of the voltage to the control
surfaces 30 and 34 will cause a charging current to flow briefly in the
control circuit (consisting of the first and second control surfaces 30
and 34 and the first and second control voltage supplies 21 and 22). This
charging current can be thought of as charging the capacitor C3 formed by
the first and second control surfaces 30 and 34, respectively. The
difference in potential between the control voltages V.sub.c1 and V.sub.c2
must be above the minimum and below the maximum for the physical structure
of the MEM switch if proper operation is to occur. This minimum voltage
and charge is dependent on the physical parameters of the switch and
variations in the fabrication process. It is normally determined
experimentally for each and every switch. Typical values are 5 to 50
volts.
Similarly, the maximum voltage and change is also determined
experimentally, although it is high enough above the minimum that it is
typically not measured, since measurement (and resultant arc) may have the
side effect of destruction of the switch elements.
As per the conventional MEM switch 10' of FIG. 1, the speed of closure of
the switch 10 can be controlled to some degree by controlling the
magnitude of the charges on the control surfaces. The initial charges
placed on the control surfaces will have a maximum value set primarily by
arc limitations between the control surfaces and the remaining elements of
the MEM switch. This maximum charge will apply a maximum coulomb force to
the cantilever beam, resulting in a maximum initial closure rate for the
beam. As the beam approaches the control surface during closure, the
capacitance will increase. While application of a constant voltage will
result in an increase in the surface charge (through Q=CV) and a resultant
increase in the coulomb force beyond the square-law effect, the voltage
may exceed the capabilities of the beam to control surface spacing. Thus,
it may be necessary to reduce the control voltage as the switch closes to
prevent a discharge arc of the stored charge. This leads to a decayed
voltage wave shape versus time for the fastest MEM switch closure
operation. The shape of the control voltage decay for the fastest closure
depends on the constructional details of the MEM switch 10. When the MEM
switch achieves closure, the control voltage must remain between the
maximum closed voltage (determined by arc or cantilever deflection) and
the minimum voltage needed to provide sufficient coulomb force needed to
overcome the cantilever spring restoration force. The optimum control
voltage driving waveform for MEM switch closure depicted in FIG. 3 is
applicable to the improved MEM switch 10 of the present invention as well.
For the purposes of switch closure (only), it is possible to set the first
control voltage, V.sub.c1, equal to zero. This is equivalent to short
circuiting capacitor C1. The action of the charge distribution then
becomes identical to the conventional MEM switch case, and second control
voltage, V.sub.c2, (relative to the rest of the MEM switch structure)
determines the closure operation of the improved MEM switch 10. Setting
the control voltage to zero also has the secondary benefit of causing the
first control surface 32 to act as an electrostatic shield for the
cantilever beam 20. This may be useful in low level signal applications
where the control signal could be considered as a potential source of
interference to the switched signal. Under such conditions, 16 must be an
insulator and V.sub.L must be connected directly to 20, with no connection
to V.sub.c1 or V.sub.c2.
It is also possible to reverse the above case, and set the second control
voltage V.sub.c2, equal to zero. This is a mode that is available only in
the improved MEM switch 10 of the present invention. The operation is
otherwise identical to the conventional case. The electrostatic shield
offered by the first control surface 30 is no longer applicable for this
case.
The MEM switch is held in the closed position by a continuous application
of the electrostatic attraction force. This usually means maintaining the
control voltages V.sub.c1 and V.sub.c2 for the duration of the closure
time, thus maintaining the charges on the control surfaces responsible for
the coulomb attractive force.
There are a large variety of values and waveforms for the first and second
control voltages V.sub.c1 and V.sub.c2 that will cause the MEM switch 10
to close. FIG. 7 depicts one of the many possible drive waveforms for the
improved MEM switch 10 of the present invention. In FIG. 7, both the
closing and opening process waveforms are shown.
The process for opening the improved MEM switch of the present invention is
new and novel. The control voltages V.sub.c1 and V.sub.c2 are applied to
the first and second control surfaces 30 and 34 respectively and are set
equal in magnitude and polarity and non-zero to create a repulsion
electrostatic charge that aids in opening the switch. In the conventional
MEM switch 10' the control voltage is just set to zero to remove the
electrostatic charge. In the simplest configuration, either V.sub.c1 or
V.sub.c2 is reversed in polarity with respect to the control voltages used
to close the improved MEM switch 10. This places charges of the same
polarity on the two control surfaces 30 and 34, and creates a repulsion
electrostatic coulomb force pushing the cantilever beam away from the
conductive contact.
This repulsion force adds directly to the spring restorative force of the
cantilever beam 20 and pushes the cantilever beam away from the contact 14
more rapidly than the spring restorative force alone. Depending on the
design configuration of the MEM switch 10, the repulsion force can be
significantly larger than the spring restorative force, resulting in a
switch design that is significantly more resistant to contact "stiction"
problems and which opens very rapidly.
With respect to the need to avoid excess voltage to prevent arcing, it
should be noted that in the process of opening the improved MEM switch,
there cannot be an arc between the two control surfaces if they are at the
same or nearly the same potential and polarity. Thus, the opening voltage
is limited by the arc voltage to other structures of the MEM switch 10
rather than between the control surfaces as was noted for the conventional
MEM switch 10'.
FIG. 7 provides an example of the control waveforms appropriate to open the
improved MEM switch. The waveforms shown are two of a variety of waveforms
that are possible within the scope of the present teachings. Some broad
limitations can be placed on the waveforms as shown. The voltages "Max Vc"
and "End Vc" of either polarity must not result in arcing to any element
of the MEM switch 10. During the closing operation as shown, control
voltages V.sub.c1 and V.sub.c2 must be of opposite polarity. Optimum
attraction coulomb force will result if the two voltages V.sub.c1 and
V.sub.c2 are maximum, equal, and opposite polarity. During the opening
operation, control voltages 1 and 2 must be the same polarity. Optimum
repulsion force will exist if the two voltages are maximum, equal, and the
same polarity. The start and end of the closure and opening process are
indicated in the figure.
Although FIG. 7 shows D.C. (direct current) voltages as the driving
waveform, A.C. (alternating current) voltages may also be used. The use of
A.C. driving voltages will cause a constant charging, discharging, and
opposite polarity recharging of the equivalent capacitors C1, C2, and C3.
A.C. waveforms may be useful in combating dielectric polarization that
reduces the coulomb force available. The driving waveforms should always
hold the correct polarity structure on control surfaces 30 and 34.
FIG. 7 also shows an optional discharge operation following the switch
opening process. The purpose of this discharge operation is to remove the
electrostatic coulomb repulsive force applied to the cantilever beam when
it is in the open position. If the coulomb force remains applied to the
beam 20, the cantilever beam will be pushed past the open rest position
(the neutral force open position) to a position of greater opening. The
greater opening will enhance the voltage that the switch can withstand
when open. It will also increase the cantilever spacing from the control
surface attached to the substrate and the spacing to the conductive
contact. The increased spacing will result in greater signal path
isolation and a reduced initial coulomb attractive force during the
closing process. The increased spacing will also force a longer time for
the switch to close. The increased closing time is somewhat compensated by
the cantilever spring restorative force pushing the beam back to the
neutral open position. There exists a trade-off between the switch closure
time and the switch open standoff voltage rating (and isolation) as a
result of the coulomb repulsive force available in the improved MEM switch
10.
Although a linear discharge waveform in shown in FIG. 7, any appropriate
means of discharge may be used. Generally, it may be optimum to allow the
charges on each control surface to decay in the same proportion as a
function of time. This will result in a reduction in the magnitude of the
coulomb force without encountering secondary effects that may result from
discharging the two control surfaces in unequal time frames.
FIG. 3 previously presented an optional waveform to enhance the speed of
closure by utilizing a higher initial voltage at the start of the closing
operation, and reducing the voltage as the closing operation progresses to
an end closure voltage. That waveform is also shown in FIG. 7 on control
voltage V.sub.c1. Control voltage V.sub.c2 is shown with an optional
waveform that is constant during closure. Either waveform can be used in
any combination desired for control voltages V.sub.c1 and V.sub.c2. There
are other waveforms also acceptable for switch closure, including the
previously mentioned A.C. waveforms. If the waveforms of control voltages
V.sub.c1 and V.sub.c2 both contain the increased initial voltage (+Max Vc
and -Max Vc at the start of closure) the limitation against arcing must
continue to apply.
The opening operation can potentially benefit from the same increased
voltage in the case of the improved MEM switch 10. The use of the
increased voltage is shown on control voltage V.sub.c1, but it may also be
used on control voltage V.sub.c2 if desired, subject to the limitation on
arcing to other structures in the MEM switch. As shown, control voltage
V.sub.c2 uses a constant voltage during the opening operation, and this
voltage is equal to the voltage used to close the switch. Control voltage
V.sub.c1 starts the opening process at the "end Vc" voltage of the closing
operation. As the charge on the equivalent capacitor C1 is reversed, the
voltage may be (rapidly) increased to the arc-limited value "Max Vc". As
the cantilever beam 20 draws away from the conductive contact 14, the
spacing between the two control surfaces 30 and 34 will increase. Since
the two control surfaces 30 and 34 are now at the same polarity, the
maximum voltage is now determined by arc limitations to other MEM switch
structures. The shape of the voltage increase is bounded by the arc
voltage, and for example may be determined by the discharge and opposite
polarity recharge between the two control surfaces 30 and 34.
FIG. 7 also assumes'that the maximum allowable voltage on the first control
surface 30 is "+Max Vc" or "-Max Vc", which defines the voltage peak
values shown in FIG. 7.
By way of example, and with reference to FIGS. 6 and 7, closure is
initiated by applying +50 volts to control surface 30 relative to 20, 16,
18 and by applying -50 volts to control surface 34 relative to 20, 16, 18
which creates an attractive coulomb force between 30 and 34 based on 100
volts. The charges will be determined by Q=CV for the (30-20) and (34-18)
capacitor pairs and the (30-34) capacitor.
Opening is initiated by applying -50 volts to control surfaces 30 relative
to 20, 16, 18 and by leaving 34 and -50 v which creates a repulsive
coulomb force between 30 and 34 based on the "equal" and like polarity
charges on those surfaces. There is a potential difference between (30,
34) and the other components of the switch.
Thus, the present invention has been described herein with reference to a
particular embodiment for a particular application. Those having ordinary
skill in the art and access to the present teachings will recognize
additional modifications applications and embodiments within the scope
thereof.
It is therefore intended by the appended claims to cover any and all such
applications, modifications and embodiments within the scope of the
present invention.
Accordingly,
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